In recent years, refinements in manufacturing methods and economies of scale have made solid-state lighting (SSL) a competitive alternative to more conventional lighting technologies, such as incandescent and fluorescent lighting. SSL is a superior lighting technology, primarily because it employs light-emitting diodes (LEDs) as a source of illumination. LEDs are inexpensive to manufacture, durable, and have a significantly longer lifetime compared to the lighting sources used in more conventional lighting technologies.
An LED is a p-n junction diode made of a binary or ternary group III-V semiconducting material, such as gallium phosphide (GaP) or indium gallium nitride (InGaN), for example. When a sufficient forward voltage Vf is applied across the p-n junction, electrons and holes recombine to form photons (i.e., light). This phenomenon, known as “electroluminescence,” produces light of a wavelength that is determined by the energy band gap of the particular semiconducting material being used.
In some lighting applications it is necessary or desirable for an SSL system to produce a large amount of light in one circumstance and to be dimmed to a significantly lower level in another circumstance. In a movie theater, for example, it is usually desirable to produce a large amount of light during intermissions, so that adequate lighting is available to assist moviegoers as they enter and exit the theater, and to dim the light to low levels when moviegoers are seated and movies are being presented. Because each LED can produce only a limited amount of light (usually on the order of 100 lumens or less), a plurality of LEDs is often employed in order to produce the large amount of light that is desired at the high end of the lighting range. Typically, the plurality of LEDs are connected in series (i.e., as a “string” of LEDs) or are configured in multiple strings and connected parallel (i.e., in a series-parallel configuration). So that the LEDs are able to conduct and emit light, the power supply voltage required to power each LED string must be greater than the sum of the forward voltage drops Vf across all LEDs in the string. Because the forward voltage Vf across each LED necessary for the LED to conduct and emit appreciable light is typically within the range of 2 to 4 volts, the required power supply voltage can be high when a large number of LEDs are connected in the string.
To dim the light produced by the LED string to the lower end of the lighting range, a dimmer of some kind is used. Typically, the dimmer is an electrical circuit which employs a technique known as pulse width modulation (PWM). PWM is a process in which a PWM control signal comprising a periodic sequence of pulses is used to intermittently interrupt current flowing through the LED string. By controlling (i.e., modulating) the durations (i.e., “widths”) of the pulses in the PWM control signal, the fraction of time during each period that the LEDs can conduct and thereby emit light can be controlled. To perform dimming, the durations of the pulses are simply reduced in duration so that the LEDs conduct and emit light less often. On average, the human eye perceives this effect as dimming.
Although PWM is a viable approach to dimming, the high supply voltages that are necessary to power the LED strings can potentially damage electrical components in the SSL system, particularly those components that are connected in series with the LED string. This problem is particularly acute at low dimming levels when the LEDs are not conducting for large fractions of the PWM control periods.